A protractor is a rectangular, square, triangle, circular, oval or semicircular measuring instrument, typically made of transparent plastic or glass, for measuring angles. Most protractors measure angles in degrees (°). Radian-scale protractors measure angles in radians.They are used for a variety of mechanical and engineering-related applications, but perhaps the most common use is in geometry lessons in schools.Some protractors are simple half-discs. More advanced protractors, such as the bevel protractor, have one or two swinging arms, which can be used to help measure the angle.